Such a Brillouin ring laser is known from European patent 0,104,942.
Ring laser serve as inertial angular rate sensors. Due to the Sagnac effect, a rotary movement of a ring laser causes the length of the resonance cavity to become different for clockwise and counter-clockwise propagating light waves. Accordingly, different frequencies of the ring lasers result for the clockwise and counter-clockwise light waves. These frequencies can be superimposed and can be caused to provide interferences at a detector. A signal at the beat frequency of the two ring laser frequencies will then be provided by the detector. This beat frequency is proportional to the angular rate. With conventional ring lasers, however, there will be a range of small angular rates in which the ring laser "locks-in" at the same frequency for both directions of propagation due to back scattering.
This "lock-in" is avoided with so-called "Brillouin" ring lasers. These Brillouin ring lasers comprise a ring resonator into which clockwise and counter-clockwise propagating light waves are "pumped" by means of a laser source. At a certain energy level of the pumped-in light waves, acoustic waves are generated in the ring resonator by the light waves. These acoustic waves propagate in the directions of the respective light waves. The acoustic waves cause variations of the refractive index and generate Brillouin light waves. These Brillouin light waves are light waves scattered at the acoustic waves. The Brillouin light waves propagate in a direction opposite to the direction of the acoustic waves and of the light waves from the laser source. The Brillouin light waves exhibit a frequency shift relative to the light waves from the laser (U.S. Pat. No. 4,159,178). Also the thus generated Brillouin light waves propagating in opposite directions are subjected to the Sagnac effect and can be utilized for measuring angular rate in the manner described above.
In European patent 0,104,942, a single continuous fiber loop is provided. The fiber loop forms an intersection with itself. Thereby the closed median portion of the fiber loop provides a resonator ring. The ends of the loop are joined by a four-port coupler. A second four-port coupler couples the beginning and the end of the resonator ring at the intersection. The first free end of the fiber loop extends to a laser source. The second free end of the fiber loop extends to a detector. Light from the laser source is coupled from the first end of the fiber loop into the second end. Thereby, light is directed to the resonator ring through both ends. The light waves supplied to the resonator ring from the two ends propagate in the resonator ring in opposite directions. Brillouin light waves are generated in the manner described above. By means of the second coupler, the Brillouin light waves from the two directions of propagation are coupled-out again into a respective one of the ends of the fiber loop. The coupled-out Brillouin light waves are superimposed by the first coupler and provide a beat signal at the detector.
The prior art Brillouin ring laser provides a beat frequency which is proportional to angular rate. The beat frequency, however, does not provide the direction of rotation.
German patent application 3,805,904 describes a fiber optical gyro operating on the basis of optical interferences and having a fiber loop. The two ends of the fiber loop and a supply fiber for supplying the light are passed through a six-port coupler. The supply fiber receives light from a light emitting diode. The ends of the fiber loop extend to photodiodes. The light emitting diode generates in the fiber ring, through the six-port coupler, clockwise and counter-clockwise light waves. The photodiodes are exposed, again through the six-port coupler, to these two light waves. An optical interference will be observed at the photodiodes. If the fiber loop is subjected to an angular rate about an axis perpendicular to its plane, the optical path lengths for the clockwise and counter-clockwise propagating light waves will be changed in opposite directions due to the Sagnac effect. This causes a corresponding change of the phases of the light waves interfering at the photodiode, and thereby a change of the light intensity. These changes of intensity have opposite directions at the two photodiodes arranged at the two ends of the fiber loop, depending on the direction of the rotation. Thus the relative intensity changes at the two photodiodes permit conclusions on the phase change and, thereby, on the angular rate and direction of rotation.
This is not a laser with a resonance. There are no frequency variations but a phase shift of the light. There are no A.C. signals at the photodiodes.